Hard disagree. Audio mixing is not difficult[1]. The Linux kernel guys were right - it does not belong in the kernel. The userspace story however, has been a complete shitshow for decades. I think Pipewire mostly fixed that? Not sure, sometimes I still have to log out and back in to fix audio.
The funniest part? It's been working in the BSDs all along. I recommend reading the source of sndiod[1].
[1]: <https://cvsweb.openbsd.org/src/usr.bin/sndiod/>
What's even worse? Probably systemd. I try not to hold a strong opinion - I tolerate it, the way I tolerate traffic when taking a walk. The technical issue however is several orders of magnitude simpler - again, the BSDs come to mind, but you can also write a complete PID1 program in about 20 lines of straightforward C[2]. I don't mind the commands being unfamiliar (they're already all different in almost every OS family); it's that the entire package is dreadfully large in scope, opaque, and I find it more difficult to interact with than anything else in this landscape.
[2]: <https://ewontfix.com/14/>